This invention relates to a gas venting arrangement for use with a molding machine, such as one used in die-casting operations.
In die-casting operations a quantity of molten metal is injected into a die cavity which has a shape corresponding to the shape of the final die-cast product. In order to ensure a substantially void-free die casting, the mold cavity is placed under vacuum throughout the melt injection process to remove as much gaseous material as possible from the mold cavity. One type of commonly used gas evacuation or venting arrangement incorporates a shut-off valve for controlling gas flow out of the mold cavity through a gas vent passage. The valve remains open during injection and, when the mold cavity is filled with melt, the valve closes to prevent the vacuum source from ingesting any excess melt which may flow out of the mold and through the gas vent passage.
Valved gas venting devices of the prior art generally are of two types, both typically involving a reciprocating valve body which cooperates with an annular seat. In one type the movement of the valve body to its closed position is accomplished by a positive drive mechanism which is synchronized with melt injection so as to close when the mold cavity is full. Morton, U.S. Pat. No. 3,121,926, discloses one example of this type of arrangement wherein a trigger switch actuated by the injection ram initiates valve closure. Thurner, U.S. Pat. No. 4,463,793 discloses another example of this type of arrangement. These types of positively driven gas vent valves, while effective, are not foolproof, inasmuch as rapidly advancing melt in some instances may reach the valve body before it is completely closed, thereby fouling the valve and seat, preventing full closure and clogging the evacuation system.
The other type of valve arrangement used in the prior art involves a melt-driven valve body wherein movement of the valve body to its closed position is effected by pressure of dynamic forces exerted by the melt itself directly on the valve body. In many situations, such as that disclosed by Takeshima, U.S. Pat. No. 4,431,047, the valve body is spring-biased to its open position, dynamic melt forces supposedly being sufficient to overcome the spring force and move the valve body to its closed position. However, as recognized by Takeshima in his later U.S. Pat. No. 4,489,771, such an arrangement is not foolproof because the valve body can oscillate due to serial impingement of discontinuous melt (i.e. having one or more voids) which momentarily relieves pressure on the valve body and allows it to re-open. Momentary opening of the valve in the presence of the melt can lead to fouling of the sealing surfaces and invasion of melt into the evacuation chamber. Accordingly, Takeshima in U.S. Pat. No. 4,489,771 provides a complex biasing and triggering mechanism which reverses the bias on the valve body (i.e. towards its closed position) upon initial impingement of the melt. While this arrangement may preclude valve body oscillation, it is not clear whether the closing spring force on the valve body is sufficient in all instances to seal the valve body against the valve seat if some melt has reached the valve seat and interferes with closure. Hodler, U.S. Pat. No. 3,885,618 discloses a melt-actuated valve body which floats freely in the valve chamber (when the mold is closed). Hodler relies on the configuration of the valve body to make an effective seal when it retracts within the seat, but there is no guarantee that the pressure of the melt itself acting on the valve body can overcome any obstruction that may be present between the sealing surfaces caused by solidified melt which may have splashed past the valve body and into the seat.